Answer: B. To convert sunlight and energy into food is the end result of photosynthesis.
The infinitive phrase that functions as a subject in the sentence above is 'To convert sunlight and energy into food' because it presents the verb in the infinitive form, which is the particle 'to' plus the verb 'convert' (to convert), and the objects (sunlight and energy). An infinitive phrase is the infinitive form of a verb combining with objects, complements and modifiers.